[Amps] Glow, gettering, and voltage

[Jim Thomson]

Date: Sat, 7 Jan 2017 09:01:08 -0800
From: Catherine James <catherine.james at att.net>
To: Amps Group <amps at contesting.com>
Subject: [Amps] Glow, gettering, and voltage

I've noticed an interesting effect with the 3-500Z tubes in my SB-220. 

When I run a kilowatt SSB on the low voltage setting on low HF bands, there is very little glow. When I run the same power on the high voltage setting (using less exciter drive), there is a noticeable dull orange glow.

I cannot think of a reason why higher voltage operation would be less efficient. Thoughts? 

73,
Cathy 
N5WVR

###  two reasons.
1- with the B+  being 41 %  higher, the idle current will also be higher.   Higher idle current x higher B+    =  lotsa more idle power.
2- IF  you run the same po in both settings, the plate current will be a lot less in the higher B+  setting.   Your plate load Z will have skyrocketed. 
With higher plate load Z, the tank coil has to have twice the uh.   The tune + load caps have to be reduced to one half their values.  Since that is not
done, your  resulting tank circuit Q  will have doubled..... like from  12  to 24.   Sky high loaded Q = lousy efficiency.   Lousy eff = higher plate
dissipation.    Combined with higher idle power =  bad news. 

##  actually the higher idle current is not the issue, only when  keyed, and not driven. 

##  The SB-220 and other similar amps like the TL-922,  drake L4B etc, all had 2 x B+  positions.  Back then the  rules were  1 kw dc input on cw mode,
and 2 kw pep input on ssb.   Back then the only legal way to tune up the amp to 1200-1300 w out  was either into a dummy load, or use the lower B+  position,
less drive and less plate current.   Then when the B+ was switched to the higher position, and drive increased a bit, the amp was still tuned correctly, with no further
tune and load adjustments.  Reason is, the plate load  Z did  not change.     Tank Q  remains the same in both cases. 

##  EG:   per the manual, the Drake amp is tuned to 565 ma  @   1770 vdc.... under load.   =  1 kw dc input.   Switch to  ssb position, increase drive a bit, and now its  800 ma  @  2500 vdc  = 2 kw dc input.
2500 / 1770  = 41 %  increase.     IOW   square root of 2 = 1.414      You will notice that in the case of all those old amps that had a CW  /   SSB position, that the  higher B+
was always  41%  higher than the CW position. 

##  On any of those amps, running low power, like 1 kw dc input, but using the SSB  /  higher  B+  position always resulted in lousy eff..... tank Q  has doubled.   
Try this.  If you have a known reasonably accurate wattmeter, measure PO  and also  loaded plate current and loaded B+   using both ur low + high  B+ settings,
but running the same PO in both cases.   Then calculate the dc input.  Then calculate the tank eff  using PO  /  dc input.   Run a brief cxr to do the tests... just long enough to
take some steady state  loaded B+  /  plate current  / PO   / grid current readings. 

Jim   VE7RF